An alternator typically comprises a rotor mounted on a rotating shaft and disposed concentrically relative to a stationary stator. The rotor is typically disposed within the stator. However, the stator may be alternatively positioned concentrically within the rotor. An external energy source, such as a motor or turbine, commonly drives the rotating element, directly or through an intermediate system such as a pulley belt. Both the stator and the rotor have a series of poles. Either the rotor or the stator generates a magnetic field, which interacts with windings on the poles of the other structure. As the magnetic field intercepts the windings, an electric field is generated, which is provided to a suitable load. The induced electric field (which is commonly known as a voltage source) is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. The induced current is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. In some instances, a regulated DC output signal is applied to a DC to AC inverter to provide an AC output.
Conventionally, alternators employed in motor vehicle applications typically comprise: a housing, mounted on the exterior of an engine; a stator having 3-phase windings housed in the housing, a belt-driven claw-pole type (e.g. Lundell) rotor rotatably supported in the housing within the stator. However, to increase power output the size of the conventional alternator must be significantly increased. Accordingly, space constraints in vehicles tend to make such alternators difficult to use in high output, e.g. 5 KW, applications, such as for powering air conditioning, refrigeration, or communications apparatus.
In addition, the claw-pole type rotors, carrying windings, are relatively heavy (often comprising as much as three quarters of the total weight of the alternator) and create substantial inertia. Such inertia, in effect, presents a load on the engine each time the engine is accelerated. This tends to decrease the efficiency of the engine, causing additional fuel consumption. In addition, such inertia can be problematical in applications such as electrical or hybrid vehicles. Hybrid vehicles utilize a gasoline engine to propel the vehicle at speeds above a predetermined threshold, e.g. 30 Kph (typically corresponding to a range of RPM where the gasoline engine is most efficient). Similarly, in a so-called “mild hybrid,” a starter-generator is employed to provide an initial burst of propulsion when the driver depresses the accelerator pedal, facilitating shutting off the vehicle engine when the vehicle is stopped in traffic to save fuel and cut down on emissions. Such mild hybrid systems typically contemplate use of a high-voltage (e.g. 42 volts) electrical system. The alternator in such systems must be capable of recharging the battery to sufficient levels to drive the starter-generator to provide the initial burst of propulsion between successive stops, particularly in stop and go traffic. Thus, a relatively high power, low inertia alternator is needed.
In general, there is in need for additional electrical power for powering control and drive systems, air conditioning and appliances in vehicles. This is particularly true of vehicles for recreational, industrial transport applications such as refrigeration, construction applications, and military applications.
For example, there is a trend in the motor vehicle industry to employ intelligent electrical, rather than mechanical or hydraulic control and drive systems to decrease the power load on the vehicle engine and increased fuel economy. Such systems may be employed, for example, in connection with steering servos (which typically are active only a steering correction is required), shock absorbers (using feedback to adjust the stiffness of the shock absorbers to road and speed conditions), and air conditioning (operating the compressor at the minimum speed required to maintain constant temperature). The use of such electrical control and drive systems tends to increase the demand on the electrical power system of the vehicle.
Similarly, it is desirable that mobile refrigeration systems be electrically driven. For example, driving the refrigeration system at variable speeds (independently of the vehicle engine rpm) can increase efficiency. In addition, with electrically driven systems the hoses connecting the various components, e.g. the compressor (on the engine), condenser (disposed to be exposed to air), and evaporation unit (located in the cold compartment), can be replaced by an electrically driven hermetically sealed system analogous to a home refrigerator or air-conditioner. Accordingly, it is desirable that a vehicle electrical power system in such application be capable of providing the requisite power levels for an electrically driven unit.
There is also a particular need for a “remove and replace” high power alternator to retrofit existing vehicles. Typically only a limited amount of space is provided within the engine compartment of the vehicle to accommodate the alternator. Unless a replacement alternator fits within that available space, installation is, if possible, significantly complicated, typically requiring removal of major components such as radiators, bumpers, etc. and installation of extra brackets, belts and hardware. Accordingly, it is desirable that a replacement alternator fit within the original space provided, and interfaces with the original hardware.
In general, permanent magnet alternators are well known. Such alternators use permanent magnets to generate the requisite magnetic field. Permanent magnet generators tend to be much lighter and smaller than traditional wound field generators. Examples of permanent magnet alternators are described in U.S. Pat. Nos. 5,625,276 issued to Scott et al on Apr. 29, 1997; 5,705,917 issued to Scott et al on Jan. 6, 1998; 5,886,504 issued to Scott et al on Mar. 23, 1999; 5,929,611 issued to Scott et al on Jul. 27, 1999; 6,034,511 issued to Scott et al on Mar. 7, 2000; and 6,441,522 issued to Scott on Aug. 27, 2002.
Particularly light and compact permanent magnet alternators can be implemented by employing an “external” permanent magnet rotor and an “internal” stator. The rotor comprises a hollow cylindrical casing with high-energy permanent magnets disposed on the interior surface of the cylinder. The stator is disposed concentrically within the rotor casing, and suitably comprises a soft magnetic core, and conductive windings. The core is generally cylindrical width an axially crenellated outer peripheral surface with a predetermined number of equally spaced teeth and slots. The conductive windings (formed of a suitably insulated electrical conductor, such as varnished copper motor wire), are wound through a respective slot, outwardly along the side face of the core around a predetermined number of teeth, then back through another slot. The portion of the windings extending outside of the crenellation slots along the side faces of the core are referred to herein as end turns. Rotation of the rotor about the stator causes magnetic flux from the rotor magnets to interact with and induce current in the stator windings. An example of such an alternator is described in, for example, the aforementioned U.S. Pat. Nos. 5,705,917 issued to Scott et al on Jan. 6, 1998 and 5,929,611 issued to Scott et al on Jul. 27, 1999.
The power supplied by a permanent magnet generator varies significantly according to the speed of the rotor. In many applications, changes in the rotor speed are common due to, for example, engine speed variations in an automobile, or changes in load characteristics. Accordingly, an electronic control system is typically employed. An example of a permanent magnet alternator and control system therefore is described in the aforementioned U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997. Examples of other control systems are described in U.S. Pat. No. 6,018,200 issued to Anderson, et al. on Jan. 25, 2000. Other examples of control systems are described in commonly owned co-pending U.S. patent application Ser. No. 10/860,393 by Quazi et al, entitled “Controller for Permanent Magnet Alternator” and filed on Jun. 6, 2004 and No. 11/347,777 by Faber man et al (including the present inventors), entitled “Controller for AC Generator” and filed Feb. 2, 2006. The aforementioned commonly owned applications are hereby incorporated by reference as if set forth verbatim herein.
The need to accommodate a wide range of rotor speeds is particularly acute in motor vehicle applications. For example, large diesel truck engines typically operate from 600 RPM at idle, to 2600 RPM at highway speeds, with occasional bursts to 3000 RPM, when the engine is used to retard the speed of the truck. Thus the alternator system is subject to a 5:1 variation in RPM. Light duty diesels operate over a somewhat wider range, e.g. from 600 to 4,000 RPM. Alternators used with gasoline vehicle engines typically must accommodate a still wider range of RPM, e.g. from 600 to 6500 RPM. In addition, the alternator must accommodate variations in load, i.e., no load to full load. Thus the output voltage of a permanent magnet alternator used with gasoline vehicle engines can be subject to a 12:1 variation. Accordingly, if a conventional permanent magnet alternator is required to provide operating voltage (e.g. 12 volts) while at idle with a given load, it will provide multiples of the operating voltage, e.g. ten (10) times that voltage, at full engine RPM with that load, e.g. 120 volts. Where the voltage at idle is 120 V, e.g. for electric drive air conditioning, or communications apparatus, the voltage at full engine RPM would be, e.g. 1200 volts. Such voltage levels are difficult and, indeed, dangerous to handle. In addition, such extreme variations in the voltage and current may require more expensive components; components rated for the high voltages and currents produced at high engine RPM (e.g. highway speeds) are considerably more expensive, than components rated for more moderate voltages.
The stator of a conventional high current motor vehicle alternator is constructed with conductors of large cross sectional area effectively connected in series. More particularly, coil groups, one associated with each phase (the A, B and C Phase) are conventionally employed. The respective Phase coil groups, (A, B and C) are connected together (terminated) as a ‘WYE’ or ‘Delta’ at one end. The opposite ends of the coil groups are arranged by phase so that each phase is isolated and then terminated to both collect and exit the alternator to a voltage control. On the exiting termination end, the coil ends of like phases are soldered in groups to insulated motor lead wire. These motor lead wires may then in turn be soldered in groups to even larger gauge motor lead wire culminating in three separate conductors for each phase, A, B and C. The lead wires are then secured to the stator by lashing the conductors to the end turns of the stator. Lashing conductors to end turns reduces the amount of exposed copper to cooling fluid passing through the alternator, in effect acting as an insulating blanket and hindering cooling of the end turns and lead wires. Several additional problems can exist with this winding method. For example: because of the low number of turns (in some instances only a single turn) per pole phase coil, it is difficult or impossible to make a small change in design output voltage by changing the number of turns of the phase pole coil; the large cross sectional area of the conductors make the stator difficult to wind; and a short circuit between coils will typically burn out the entire stator and may stall the alternator, resulting in possible damage to the drive system or overloading the vehicle engine.
In general, permanent magnet alternators incorporating a predetermined number of independent groups of windings, wound through slots about predetermined numbers of teeth where the power provided by each group is relatively unaffected by the status of the other groups are known. For example, such an alternator is described, together with a controller therefor, in U.S. Pat. No. 5,900,722 issued to Scott et al. on May 4, 1999. In the alternator described in U.S. Pat. No. 5,900,722, the number of groups of windings was equal to an integer fraction of the number of poles, and the controller circuit selectively completed current paths to the individual groups of windings to achieve a desired output.
However, there remains a need for a compact high power alternator wherein a desired output voltage can be achieved by changing the number of turns of the phase pole coil, that is relatively easy to wind, and minimizes the consequence of short circuits, while at the same time facilitating cooling.